WASHINGTON — President Trump traveled to Detroit on Wednesday to announce a rollback of stringent fuel economy standards for cars and trucks that were put in place by the Obama administration — a welcome message to American automakers but one that could slow the push for a new generation of efficient vehicles.

The fuel-economy rules, aimed at cutting heat-trapping carbon dioxide, were one of the two main pillars of President Barack Obama’s climate change legacy. Put forth in 2012, they would have required automakers to nearly double the average fuel economy of new cars and trucks to 54.5 miles per gallon by 2025, forcing automakers to speed development of highly fuel-efficient vehicles, including hybrid and electric cars.

The rules have been widely praised by environmentalists and energy economists for reducing the nation’s dependence on foreign oil and its greenhouse pollution. If put fully into effect, the fuel efficiency standards would have cut oil consumption by about 12 billion barrels and reduced carbon dioxide pollution by about six billion tons over the lifetime of all the cars affected by the regulations.

That would have been a little more than the amount of oil consumed and carbon pollution produced by the United States in a year, according to the Environmental Protection Agency.

But automakers said the fuel standards would be technologically hard to achieve and expensive to comply with. In their first Oval Office meeting with Mr. Trump, executives from the three Detroit car companies — General Motors, Ford Motor and Fiat Chrysler — asked him to revisit and revise his predecessor’s rules.

Mr. Trump is using his Detroit trip to tell the executives that he will oblige their request — and to tell autoworkers that the loosened rules will stimulate the growth of manufacturing jobs.

The president’s announcement alone will not be enough to roll back the standards; that will take more than a year of legal and regulatory reviews by the E.P.A. and the Transportation Department. The Trump administration will then propose its replacement fuel-economy standards by April of next year, according to a senior White House official.

The Motor City announcement is the first of an expected one-two punch from Mr. Trump aimed at undercutting Mr. Obama’sclimate change policies. Mr. Trump is also expected to announce in the coming weeks that he intends to direct the E.P.A. to dismantle Mr. Obama’s regulations on planet-warming pollution from coal-fired power plants.

The announcements follow public remarks last week by the E.P.A. administrator, Scott Pruitt, that he does not believe carbon dioxide is a primary driver of global warming, a statement at odds with the global scientific consensus on climate change.

“These announcements mean that Mr. Trump is going to live up to his campaign vows, reversing course on climate, destroying much of the Obama legacy in this realm, and increasing these levels of harmful emissions,” said Robert N. Stavins, director of the environmental economics program at Harvard University.

Mr. Stavins and other experts noted that these policies would make it impossible for the United States to meet Mr. Obama’s commitments under the 2015 Paris Agreement, the landmark accord that committed nearly every nation to reducing greenhouse pollution. Mr. Obama’s pledge that the United States would reduce its emissions about 26 percent from 2005 levels by 2025 was dependent upon the enactment of the stringent regulations on tailpipe and smokestack pollution.

But automakers have complained for months that the fuel economy standards hurt their bottom lines.

On Feb. 21, a coalition of the 17 largest companies that sell cars in the United States sent two letters to Mr. Pruitt, asking him to revisit the tailpipe rules. They said it may be “the single most important decision the E.P.A. has made in recent history.”

They complained about the steep technical challenge posed by the standard, noting that only about 3.5 percent of new vehicles are able to reach it. That even excludes some hybrid cars, plug-in electric cars and fuel cell vehicles, the automakers wrote, adding, “Even today, no conventional vehicle today meets that target.”

The automakers estimated their industry would have to spend a “staggering” $200 billion between 2012 and 2025 to comply with the rules.

While automakers are legally locked in to meeting the increasing fuel economy standards through 2021, Mr. Trump’s announcement is expected to signal that the E.P.A. and the Transportation Department will review and revise the standards for model years 2022 through 2025 — the years in which automakers would be required to most drastically increase vehicle fuel economy.

Environmental groups have challenged the automakers’ assertion that fuel-saving technology needed to meet the standards is too costly.

The research group Consumers Union has found that the current standards for 2025 will allow consumers to save enough money on gasoline to more than offset any price increases for new technology. “Fuel efficiency technology pays for itself and is a boon to car and truck buyers that benefit from the savings greater efficiency offers,” said Shannon Baker-Branstetter, policy counsel for Consumers Union.

Meanwhile, economists say, rolling back the standards would increase, rather than decrease, the nation’s oil dependence.

The standards “affect U.S. oil dependence by reducing our oil consumption,” said Richard Newell, president of Resources for the Future, a nonpartisan research organization in Washington focused on energy economics.

Mr. Trump will not be granting the industry’s request for a broad review of fuel rules free of cost: He is expected to press carmakers to increase investment and add jobs in their manufacturing operations in the United States.

During the campaign and since the November election, Mr. Trump has been critical of companies, including G.M. and Ford, for building vehicles in Mexico for the United States market.

In response, several automakers have announced plans to add jobs and ramp up investment in existing American factories. Ford went a step further and canceled a new $1.6-billion assembly plant in Mexico that had been repeatedly criticized by Mr. Trump.

But relaxed fuel-economy standards may not provide enough incentive for automakers to risk adding significant numbers of jobs.

Car companies are making big profits in the North American market, and their plants are running at near-capacity levels. Last year, auto companies set a second consecutive annual sales record in the United States with about 17.5 million new vehicles sold.

The growth in sales, however, has begun to level off. And automakers are wary of expanding production beyond the current level of demand.

Correction:

An earlier version of an article on President Trump’s decision to roll back fuel economy standards misidentified a Harvard University expert on climate change. He is Robert Stavins, not Richard Stavins.